Re: [PATCH 3.19 v4 1/2] x86, mpx: Short-circuit the instruction decoder for unexpected opcodes

[Thomas Gleixner <tglx@linutronix.de>]

On Tue, 13 Jan 2015, Andy Lutomirski wrote:

> This reduces the degree to which we're exposing the instruction decoder
> to malicious user code at very little complexity cost.
> 
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> ---
>  arch/x86/mm/mpx.c | 25 ++++++++++++++++---------
>  1 file changed, 16 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
> index 67ebf5751222..a73004330732 100644
> --- a/arch/x86/mm/mpx.c
> +++ b/arch/x86/mm/mpx.c
> @@ -230,6 +230,22 @@ static int mpx_insn_decode(struct insn *insn,
>  	 */
>  	if (!nr_copied)
>  		return -EFAULT;
> +
> +	/*
> +	 * We only _really_ need to decode bndcl/bndcn/bndcu
> +	 * Error out on anything else.  Check this before decoding the
> +	 * instruction to reduce our exposure to intentionally bad code
> +	 * to some extent.  Note that this shortcut can incorrectly return
> +	 * -EINVAL instead of -EFAULT under some circumstances.  This
> +	 * discrepancy has no effect.
> +	 */
> +	if (nr_copied < 2)
> +		goto bad_opcode;
> +	if (buf[0] != 0x0f)
> +		goto bad_opcode;
> +	if (buf[1] != 0x1a && buf[1] != 0x1b)
> +		goto bad_opcode;

These opcodes can never have a prefix? If so, then we want to add this
to the comment.

Thanks,

	tglx